Description of Jellyfish

﻿The gelatinous jellyfish are widespread in marine environments, although they are most common in tropical and subtropical regions. These ancient animals first appeared on earth over 650 million years ago. The smallest jellyfish are difficult to see without a microscope, while the largest known jellyfish is 2.5 meters in diameter; some jellyfish may have tentacles over 100 feet in length. The body plan of jellyfish is relatively primitive and contains less than 5 percent solid organic matter, the remaining bulk coming from water. They completely lack internal organs. The bell-like jellyfish bodies are composed of an outer layer of epidermis and an inner layer of gastrodermis that lines the gut. The gut has a single oral opening. Between the two layers is the mesoglea, which contains few cells and has a low metabolic rate. Four to eight oral arms are located near the oral opening and are used to transport food that has been captured by the tentacles. Jellyfish are able to exert minimal control over their movement, being largely at the mercy of ocean currents. Jellyfish do, however, have some regulation over vertical movement. They possess a ring of muscles embedded on the underside of the bell that pulses rhythmically, pushing water out of the hollow bell. Using this jet propulsion, jellyfish can change their position in the water column, moving in response to light and prey.